Micro-electro-mechanical transducers usually share a common feature, which includes a movable mechanical part used for energy transformation. One example of such micro-electro-mechanical transducers is micromachined ultrasonic transducers (MUT). An ultrasound transducer performs a chain of energy transformation to realize its function of a transducer. In its receiving mode, the acoustic energy of ultrasound waves propagating in a medium where the transducer is placed is transformed to mechanical energy of a movable part (conventionally a vibrating membrane) in the transducer. The motion of the movable part is then transformed to a detectable electromagnetic (usually electrical) signal. In its transmitter mode, the reverse chain of energy transformation takes place.
Various types of ultrasonic transducers have been developed for transmitting and receiving ultrasound waves. Ultrasonic transducers can operate in a variety of media including liquids, solids and gas. These transducers are commonly used for medical imaging for diagnostics and therapy, biochemical imaging, non-destructive evaluation of materials, sonar, communication, proximity sensors, gas flow measurements, in-situ process monitoring, acoustic microscopy, underwater sensing and imaging, and many others. In addition to discrete ultrasound transducers, ultrasound transducer arrays containing multiple transducers have been also developed. For example, two-dimensional arrays of ultrasound transducers are developed for imaging applications.
Compared to the widely used piezoelectric (PZT) ultrasound transducer, the MUT has advantages in device fabrication method, bandwidth and operation temperature. For example, making arrays of conventional PZT transducers involves dicing and connecting individual piezoelectric elements. This process is fraught with difficulties and high expenses, not to mention the large input impedance mismatch problem presented by such elements to transmit/receiving electronics. In comparison, the micromachining techniques used in fabricating MUTs are much more capable in making such arrays. In terms of performance, the MUT demonstrates a dynamic performance comparable to that of PZT transducers. For these reasons, the MUT is becoming an attractive alternative to the piezoelectric (PZT) ultrasound transducers.
Among the several types of MUTs, the capacitive micromachined ultrasonic transducer (cMUT), which uses electrostatic transducers, is widely used. Other MUTs using piezoelectric (pMUT) and magnetic (mMUT) transducers are also adopted. Examples of prior art cMUT structure are shown in FIGS. 1A-1C, FIGS. 2-3, and FIGS. 4A-4B.
FIG. 1A shows a cross-sectional view of a basic structure of a prior art cMUT having multiple cells. FIG. 1B shows an enlarged view of a single cMUT cell 10. FIG. 1C shows a corresponding schematic top view of the same prior art multi-cell cMUT structure. In practice, a functional cMUT may have at least one independently addressable cMUT element. Based on the conventional design, each cMUT element consists of many cMUT cells, which are connected in parallel. Four cells are shown in FIG. 1A, a single cell is shown in FIG. 1B, and ten cells are shown in FIG. 1C, but all cells belong to a single cMUT element in FIGS. 1A-1C.
The cMUT of FIGS. 1A-1C is built on a substrate 11. As shown in a selected cMUT cell 10, each cMUT cell has a parallel plate capacitor consisting of a rigid bottom electrode 12 and a top electrode 14 residing on or within a flexible membrane 16 that is used to transmit or receive an acoustic wave in the adjacent medium. The flexible membrane 16 in each cell is supported by the insulation wall or posts 18. The membrane 16 is spaced from the substrate 11 and the top electrode 12 to define a transducing space 19 therebetween. A DC bias voltage is applied between the electrodes 12 and 14 to deflect the membrane 16 to an optimal position for cMUT operation, usually with the goal of maximizing sensitivity and bandwidth. During transmission, an AC signal is applied to the transducer. The alternating electrostatic force between the top electrode and the bottom electrode actuates the membrane 16 in order to deliver acoustic energy into the medium (not shown) surrounding the cMUT. During reception, the impinging acoustic wave vibrates the membrane 16, thus altering the capacitance between the two electrodes. An electronic circuit detects this capacitance change.
Alternatively, the membrane can be actuated and the displacement of the membranes detected using a piezoelectric transducer (pMUT) and a magnetic transducer (mMUT). FIG. 2 shows a pMUT cell 20, which has a similar structure to the cMUT cell 10 except that the capacitor (electrodes 12 and 14) are replaced by a piezoelectric member 24 on the membrane 26. FIG. 3 shows an mMUT cell 30, which has a similar structure to the cMUT cell 10 except that the capacitor (electrodes 12 and 14) are replaced by a magnetic member 34 on the membrane 36.
Methods of fabrication for making a cMUT shown in FIGS. 1A-1C have been developed. Exemplary methods are disclosed in U.S. Pat. Nos. 6,632,178 and 6,958,255.
There are drawbacks in the cMUTs of the prior art structures and methods. Many of these drawbacks relate to the fact that each addressable cMUT element is made of many individual cells and each cell has its cMUT membrane clamped or fixed on edges shared by the adjacent cells. Examples of the drawbacks are listed below.
(1) The average displacement of the membranes is small because of the clamped edges. As a result both the device transmission and reception performance are poor.
(2) Surface areas occupied by the clamped areas (e.g., edges) and the walls or posts are non-active, thus reducing the device fill factor and the overall efficiency.
(3) Anchor areas introduce a parasitic capacitance, which decreases the device sensitivity.
(4) The anchor pattern within the surface of the cMUT element may cause ultrasonic wave interference, which limits the device bandwidth.
(5) The non-uniform displacement of the membrane may disturb the ultrasonic wave pattern. For example, the non-uniform displacement may affect the ultrasonic beam pattern emitted from the transducer surface and also cause acoustic cross coupling through the transducer surface.
(6) The resonant frequencies of individual cells in the same cMUT element may be different between each other because of the process variation. This causes phase differences of the membrane motion among different cells in the same cMUT element during operation. As a result, the sum of the average displacement of the cMUT element may degrade dramatically. This problem degrades the device performance especially when the cMUT works in a high quality factor (Q-factor) condition, for example in air.
(7) The acoustic energy can couple into the transducer substrate through supporting walls and cause undesired effects such as acoustic cross coupling between the cMUT elements. An effort to reduce the cross coupling through the substrate by introducing materials with desired acoustic properties may require occupation of extra space between elements.
The above problems also exist in the pMUT and mMUT of the prior art since they have a similar structure as the cMUT as shown in FIG. 1.
Another cMUT device having a compliant support structure built on the substrate to support the membrane is disclosed in the U.S. Pat. No. 7,030,536. A cMUT according to that design is shown in FIGS. 4A-4B. FIG. 4A shows cross-sectional view of a single cMUT cell 40 as disclosed in that patent. FIG. 4B shows the schematic top view of multiple cMUT cells as disclosed in that patent. Compared to the conventional cMUT structure shown in FIGS. 1A-1C, the cMUT structure disclosed in U.S. Pat. No. 7,030,536 uses a compliant support structure 48 in place of the conventional insulation wall 18 to define the perimeter ends of the membrane 46 of each cMUT cell 40, such that the top electrode 44 and the membrane 46 may move in a piston-like manner. This has potential advantages but the design according to the patent also introduces its own problems, as will be discussed in view of the present invention in the detailed description.
Due to the importance of these MUT devices, it is desirable to improve the technology in terms of performance, functionality, and manufacturability.